| Summary: | We have developed ultrasonic instrumentation for simultaneous flow and composition measurement in a variety of gas mixtures. Flow and composition are respectively derived from measurements of the difference and average of sound transit times in opposite directions in a flowing process gas. We have developed a sound velocity-based algorithm to compensate for the effects of additional gases, allowing the concentrations of a pair of gases of primary interest to be acoustically measured on top of a varying baseline from ‘third party’ gases whose concentrations in the multi-gas mixture are measured by other means. Several instruments are used in the CERN ATLAS experiment. Three monitor C<sub>3</sub>F<sub>8</sub>, (R218), and CO<sub>2</sub> coolant leaks into N<sub>2</sub>-purged environmental envelopes. Precision in molar concentration of better than 2 × 10<sup>−5</sup> is routinely seen in mixtures of C<sub>3</sub>F<sub>8</sub> in N<sub>2</sub> in the presence of varying known concentrations of CO<sub>2</sub>. Further instruments monitor air ingress and C<sub>3</sub>F<sub>8</sub> vapor flow (at high mass flows around 1.1 kg s<sup>−1</sup>) in the 60 kW thermosiphon C<sub>3</sub>F<sub>8</sub> evaporative cooling recirculator. This instrumentation and analysis technique, targeting binary pairs of gases of interest in multi-gas mixtures, is promising for mixtures of anesthetic gases, particularly in the developing area of xenon anesthesia.
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